A2/A-level Chemistry - Rate and Equilibrium

• In A2/A-level Chemistry, Rate and Equilibrium are taught in unit 3.1.9/10
•  Knowledge on Rate & Equilibrium help us to understand the reaction mechanism better
• Rate of reaction can be calculated from given concentration of reactants and products
• By comparing reaction quotient with equilibrium constant, we can predict the potential direction that a reaction would proceed

Rate of reaction

Definition:

- implies the relationship between rate of reaction and initial concentration

• [A] / [B]: initial concentrations of the reactants in mol dm^-3
• a/b: order of reaction with respect to reactant A/B (determined by experiment)
• k: reaction constant (varies as temperature changes)

Common keywords:

Half-life: time required to reduce reactant concentration by half

Rate-determining step: the slowest step in the reaction, determines the overall rate

Order of Reaction: the power to the concentration of the reactant, determined by using concentration-time graphs/rate-concentration graphs.

Concentration-Time graph

Types of graph:

Zero order: Conc. decreases↓ at a constant rate, half-life decreases↓

First order: Conc. halves in equal time interval, half-life remains constant

Second order: Conc. decreases↓ at depriciating rate, half-life increases↑

Rate-concentration graph

Zero order: reaction rate remains constant for any concentration

First order: [concentration of X] directly proportional to reaction rate

Second order: [concentration of X]² proportional to reaction rate

Rate constant

• usually expresses in k
• unit: depends on [A]/[B] with respect to their orders
• In A2/A level Chemistry, rate equation is defined as Rate = k[A]^a[B]^b
• e.g. Rate = k[A][B]² --> k = Rate/([A][B]²) 
• k unit =  mol dm^-3s^-1/(mol dm^-3)³ = mol^-2 dm⁶ s^-1

Effect of temperature(T) on rate constant

• reaction rate always increases↑ exponentially as temperature increases↑
• illustrate by Maxwell-Boltzmann Distribution

Expaination:

• at low T, molecules are less active
• T increases↑, kinetic energy of molecules  >= activation energy
• chance of successful collision increases↑

Rate-determining step

• the slowest reaction in the reaction mechanism which controls the rate of reaction
• Rate = k[A][B] ---> reactants in rate-determining step
• steps after RDS have no effect on reaction rate

• e.g. A + B + 2C --> P
• if reactant A is zero order, reactant B is first order, reactant C is second order
• Rate = k[B][C]²

Dynamic Equilibrium

Features:

1. forward reaction rate = backward reaction rate
2. no net change in concentrations
3. equilibrium can be reached in closed system only

Equilibrium constant (Kc)

• e.g A + 3B ---> 2C
• Kc = [products]/[reactants] = [C]²/([A][B]³)
• all reactants/products concentration are at equilibrium
• powers of species = no. of mole in balanced equation
• assumed constant temperature 
• large Kc ---> equilibrium on right (product side)
• small Kc ---> equilibrium on left (reactant side)

Question: Will Kc change when concentrations/pressure change?

Ans:

• pressure can induce change in concentration
• HOWEVER, there is NO NET CHANGE in concentration!
• system will restore to equilibrium (by shifting to left or right)

Question: Will Kc change when temperature changes?

Ans:

• system responds depending on heat releasing(exothermic) / heat absorbing(endothermic) reaction
• if forward reaction = exothermic, backward reaction = endothermic (and vice versa)
• T increases↑ ---> shift left(reactant) to absorb heat ---> Kc decreases↓
• T decreases↓ ---> shift right(product) to release heat ---> Kc increases↑ 

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Image:

• Concentration-Time Graph & Rate-concentration Graph - https://ibchem-kinetics.tumblr.com/post/119256691287/rate-expressions
• Endothermic and exothermic reaction summary table - https://ibalchemy.com/7-1/

Drafted by Yoyo (Chemistry)